Embodiments of the inventive concept relate generally to semiconductor memory devices. More particularly, embodiments of the inventive concept relate to control devices for nonvolatile memory devices and methods of operating the control devices.
Semiconductor memory devices play a significant role in a wide variety of consumer and industrial technologies, ranging from home computers to satellite equipment. Consequently, improvements in semiconductor memory technology can have a significant impact on the performance of numerous technical applications.
Semiconductor memory devices can be roughly divided into two categories based on whether or not they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Examples of volatile memory devices include dynamic random access memory (DRAM) and static random access memory (SRAM), and examples of nonvolatile memory devices include read only memory (ROM), ferroelectric random access memory (FeRAM), phase-change random access memory (PRAM), and flash memory.
Flash memory has become an especially popular form of nonvolatile memory due to its relatively low cost, low power consumption, high integration density, and ability to withstand physical shock. At the same time, however, flash memory still suffers from various shortcomings.
One shortcoming of flash memory is that it tends to wear out after a certain number of erase or program operations are performed. In other words, flash memory has limited erase or program endurance. This limitation is a consequence of the way that flash memory cells are programmed and erased. In particular, in an erase or program operation of a flash memory, a relatively large electrical field is created between a channel region and a floating gate of a selected flash memory cell. The electrical field causes electrons to cross an insulating region of the floating gate to change the selected flash memory cell from an erased state to a programmed state or vice versa. The passage of electrons places stress on the insulating region, which can cause it to break down or leak charges after a large number of program or erase operations are performed.
Another shortcoming of flash memory is that it can lose stored data with the mere passage of time. For instance, where a flash memory cell remains programmed for a long period, such as ten years, electrical charges can diffuse away from its floating gate, resulting in lost data. To address this shortcoming, a data retention operation may be performed by reading stored data from memory cells that were previously programmed, and adjusting the threshold voltages of the memory cells to account for any lost charges. The data retention operation, however, can add stress to the insulating layer of the memory cells, similar to program and erase operations.